1. Field of the Invention
The invention relates to a reactor pressure vessel for a nuclear reactor.
Such a reactor pressure vessel typically includes a lower part which is constructed as a dome, and an adjoining upper part which is constructed cylindrically. As a rule, the reactor pressure vessel is restrained by so-called fastening claws. Today, the raw material used is typically ferritic steel material which was developed for that special application and is known by the designation 20MnMoNi55. In operation, the reactor pressure vessel is at a high internal pressure, which can amount to as much as approximately 170 bar. In order to withstand that pressure, a lower part is constructed with a wall thickness of approximately 15 cm and an upper part with a wall thickness of approximately 25 cm. In the nuclear power plant, the reactor pressure vessel is surrounded by a reactor protection building (containment), which is substantially formed of concrete.
Heretofore, the conventional thinking on safety considerations assumed that there was no need to fear reactor pressure vessel failure, because of the materials and dimensions selected. However, more-intensive safety studies of nuclear energy utilization have also considered the possibility, however unlikely, of a "failure" of a reactor pressure vessel. In particular, one new reactor type, the European Pressurized water Reactor or EPR, is based on such considerations. In contrast to the earlier philosophy of safety, in that reactor type the possibility of a core meltdown accident, a so-called MCA (Maximum Credible Accident), is not rejected out of hand. Some thought has also been directed to whether steam explosions might not occur during a core meltdown, and whether in such a critical phase, suddenly produced water vapor might not cause the pressure vessel to burst. There is no question that control must be gained over such accidents, however theoretical they may be.
The point of departure of theoretical studies is this: in an overload of a thermal nature (overheating) or of a mechanical nature (overpressure), and in particular in the event of a core meltdown accident, a crack that is propagated at high speed could occur locally in the homogeneous wall of the reactor pressure vessel. The crack can then spread, out of control, to relatively large regions. The possibility exists that a relatively large region could break up, for instance the entire dome in the lower part. Something similar could happen if the lower part of the reactor pressure vessel fills with core melt, given the high internal pressure. If a part that large were to break up, a reaction surge would occur within far less then one second, and in that surge the upper part of the reactor pressure vessel could be torn from the fastening claws and be spun like a rocket upward against the inner wall of the containment. The containment must withstand such an impact. The containment must also offer sufficient resistance if the reactor pressure vessel "explodes", or in other words breaks apart into a number of relatively large or small pieces. Even with relatively heavy concrete construction, it is difficult to absorb such explosion-like effects.